does SB6141 not work with cox?

We had the Panoramic modem/router on the gigabit plan.  Then we downgraded to preferred (150mbps).  Since there was no need for the Panoramic modem, I tried to activate a SB6141 with Cox over the phone.

During the bootup sequence, the SB6141 send/receive lights turned solid green, but the internet (globe icon) light just kept flashing, before the whole cycle repeated.

I tried several things with the technician:

* Unplugged the modem, waited for a few minutes before plugging it back in

* Tried the coax in a different room

* Tried connecting my laptop directly to the modem instead of via the router

* Verified the serial number and MAC address

* Verified the work order for downgrading to preferred internet was completed

The technician said that they couldn't even send a reboot signal to the SB6141 because it was offline.

His theory is that DOCSIS 3.0 isn't supported in my area, even though DOCSIS 3.1 is.  I don't know how much sense that makes.  He believes that if I just buy a DOCSIS 3.1 modem then everything will just work, but I'm doubtful.

What are we missing here?  Is this a known issue?